Hi Eric,
I just realized there is a 4th sample rate setting associated with the serial port rate. You need to set that to 8 kHz.
Attached is an 8 kHz sample rate StereoIn-ADC-I2S-DAC talkthru-HP Out audio passthru example. Signal flow is as follows.
You simply need to just connect a wire or a jumper between the ADC_DATA and DAC_DATA on the JP6 header, and we run the ADAU1761 serial port in master 2-channel I2S mode.
-JT